According to reports, a car had been abandoned in the area and army bomb experts had been called. The force of the blast shattered windows as far as 300 yards away.

It is thought a warning had been received and that the area had been evacuated when the bomb went off.

It is still not clear if there were any casualties but the blast caused extensive damage to the town centre.

David Trimble: left to view damage

Ulster Unionist Party leader David Trimble, in whose constituency Portadown is located, said he believed the bombing was the IRA's response to not getting their own way in the peace talks.

He also blamed another bomb on Friday night in Moira, County Down, which also caused extensive damage, on the IRA.

Mr Trimble pointed out that that bomb had been in the constituency of his party colleague Jeffrey Donaldson.

The UK Government has hinted it believes the Moira bomb to be the work of a republican splinter group called the Continuity Army Council.

"We are two leading members of the UUP involved in the talks. I think this is the IRA's response to the conduct of the talks," said Mr Trimble, who left the talks at lunchtime to visit Portadown.

A bomb exploded at Moira on Friday

"I think it underlines the silliness of people who think they can be re-admitted to the talks."

He said he believed the IRA had decided some time ago that its objectives could not be achieved through the talks process because of the Unionist presence.

But he called for loyalists to remain calm and not react violently.

"Because we have prevented them using the talks the way they wanted they are now venting their spleen on our constituencies and our towns." he added.

As army helicopters circled overhead, other unionist politicians and loyalist leaders gathered at the scene.

Deputy leader of the Democratic Unionists Peter Robinson said: "This bears all the hallmarks of the IRA. It has been carried out strategically to their advantage."

He insisted the unionist community would feel revulsion at the blast. "They are finding it difficult to understand why the government is prepared to allow the IRA to maintain their credibility in the talks.

"I don't believe there can ever be a place at the negotiating table for the people who did
this."

Also at the scene was Mark Fulton, a close associate of the late LVF leader
Billy Wright whose murder in the Maze prison in December sparked a round of killings.

He said: "The people in Portadown are incensed. I would say loyalists will not be
amused at this."